1. Field of the Invention
The present invention relates to a two-component developing device, an image forming apparatus, and a stirring screw, which are used in an electrophotographic device, etc., and more particularly to a two-component developing device, an image forming apparatus, and a stirring screw, which are used for enhancing an efficiency of stirring two component developers to improve an image printing quality.
2. Description of the Related Art
Recently, the image forming apparatus used in the electrophotographic system has been widely used as an output device connected to a computer. This image forming apparatus has required a high-speed printing performance. Therefore, the developing device also has required a high performance appropriate for high-speed printing.
A general electrophotographic method used in an image forming apparatus comprises the following steps:
charging a photosensitive carrier with electricity;
exposing by light the photosensitive carrier to form a latent image thereon;
supplying developers to the photosensitive carrier to develop the latent image;
transferring a toner image on a medium; and
fixing the toners onto the medium.
In order to develop the latent image, a two-component developing device that uses toners and carriers is utilized. A toner concentration sensor is arranged in the two-component developing device, and this developing device performs to control the toner supply amount corresponds to the toner consumption for printing from a toner hopper. This is called a toner concentration control the two-component developing method enables the toners to be used by mixing and stirring the metallic-powder contained carriers with the resin toners to friction-charge these carriers and toners with electricity. Thus, a member for mixing and stirring these carriers and toners is arranged in the two-component developing device.
FIG. 22 is a constitutional view of a stirring screw in a conventional developing device, FIG. 23 is an explanatory view of the operation of the stirring screw, and FIG. 24 is a constitutional view of a stirring screw in another conventional developing device. As shown in FIG. 22, a stirring screw 100 comprises a shaft 120 and a long-diameter spiral screw blade 110 wound around the shaft 120. By rotating the screw 100, the developer 130 is stirred and moved while being in friction-contact with the blade 110. As shown in FIG. 23, by rotating the screw 100, the developer 130 is stirred-and moved while being in friction-contact with a single face of the blade 110. In this process of stirring and moving the developer 130, the supplied toners 150, which are supplied from the toner hopper, can be stirred with the developer 130 containing the carriers and the toners, and can be also charged with electricity.
On the other hand, the high-speed printer requires a large toner consumption per hour, so both the toner supply count and the toner supply amount are increased. Therefore, as shown in FIG. 22, when the stirring speed of the screw 100 is lower than the toner supply count or the toner supply amount, light-weight resin toners tend to collect upon an upper surface layer 140 of the developer 130 even by rotating the screw 100. Such toners are not fully friction-charged with electricity, so they are uncharged. These uncharged toners are easily scattered and attach to other portions except the desired latent image to lower an image printing quality.
As shown in FIG. 24, it is proposed that pins 160 should be arranged between the spiral blades 110 around the rotary shaft 120 to enhance the stirring ability. Using this constitution enables the developer 130, which is carried by the spiral blade 110, to be stirred even with the pins 160, so the stirring ability is expected to be enhanced.
This image forming apparatus requires a high performance for high-speed printing. For example, the image forming apparatus needs to have the ability of printing 400 or more sheets per minute. When an image is formed at high speeds, not only the toner consumption per hour is increased but also the toner supply amount and the toner supply count are increased. Therefore, there is a problem that using the constitution in FIG. 24 enables the toners to be stirred in the positions of the pins 160 arranged, but not to be stirred between the pins 160, so the high-speed printer cannot perform a sufficient stirring ability.
Furthermore, there is another problem that the developer 130 is subject to stress to shorten the developer""s life because the pins 160 traverse the direction in which the blade 110 carries the developer 130. For example, the coated layers of the carriers are subject to abrasion to easily lower the friction-charging ability.
It is an object of the present invention is to provide a two-component developing device, an image forming apparatus, and a stirring screw, for enhancing the stirring ability and forming an image at high speeds.
It is another object of the present invention to provide a two-component developing device, an image forming apparatus, and a stirring screw, for enhancing the stirring ability and lowering a stress exerted on the developer.
It is another object of the present invention to provide a two-component developing device, an image forming apparatus, and a stirring screw, for enhancing the stirring ability with an inexpensive constitution.
In order to this object, a two-component developing device of the present invention comprises a developer carrying member for carrying a developer containing carriers and toners, to a latent image carrier, and a stirring screw for stirring supplied toners with the developer. The stirring screw comprises a screw shaft, a spiral screw with a relatively short diameter, and another spiral screw with a relatively long diameter, wherein these two spiral screws are alternately wound around the same screw shaft.
According to the present invention, it is focused on the fact that the generation of uncharged toners due to the increased toner supply amount is caused by the event that toners are carried to the upper and lower surface layers of the developer with different concentrations, because a long-diameter spiral screw has a sufficient ability of carrying the entire developer, but does not have a sufficient ability of stirring the upper surface layer of the developer as described above. Thus, a fundamental conception of the present invention is to arrange a member for stirring the upper surface layer of the developer, between the long-diameter spiral screw blades.
The surface layer of the developer can be stirred even with the conventional rotary pins, but the rotary pins are in point-contact with the developer, have a low stirring ability, and traverse the direction in which the long-diameter spiral screw carries the developer. At the result, the developer tends to receive a stress and is deteriorated. When the number of pins is increased to enhance the stirring ability, this tendency causes the developer to receive more stress. However, according to the present invention, by arranging a short-diameter spiral screw as a member for stirring the surface layer of the developer, the developer can be stirred while being carried on the face of a short-diameter spiral screw blade. Therefore, the short-diameter spiral screw can fully stir the surface layer of the developer without exerting a stress on the developer, so that the generation of uncharged toners can be prevented and the high-speed image printing quality can be improved.
According to the present invention, the outer diameter of the short-diameter spiral screw blade is shorter 10% to 80% than that of the long-diameter spiral screw blade. Therefore, the surface layer of the developer can be stirred in a wide range of the developing device.
Furthermore, according to the present invention, the outer diameter of the short-diameter spiral screw blade is shorter 10% to 20% than that of the long-diameter spiral screw blade. Therefore, the surface layer of the developer can be fully stirred in a range of developer amount fluctuation limited within the developing device.